Modelling the downstream longitudinal effects of frequent hydropeaking on the spawning potential and stranding susceptibility of salmonids.

Hydropower plant operating conditions are expected to change to be more in tandem with intermittent power production so as to meet the requirements of the Paris Agreement, which in turn may negatively impact ecological conditions downstream of the hydropower plants. The current study investigates how highly flexible hydropower operating conditions may impact several salmonid species (European grayling, Atlantic salmon and brown trout) in the River Umeälven, a major river in northern Sweden; specifically, how changes in hydropeaking frequency may affect the area of the downstream watercourse that is hydraulically suitable for spawning (potential spawning area) and how changes in spill gate closing time may affect the propensity to stranding. River hydrodynamics were modeled using the open-source solver Delft3D, with a range of hydropeaking frequencies (from 10 to 60 starts and stops per day) and a range of spill gate closing times from (1-30 min). Increasing the hydropeaking frequency caused a reduction in potential spawning area, but also a reduction in dewatering of potential spawning area at low flows. Increasing spill gate closing time caused a decrease in propensity to stranding. Effects were dependent on both species and life-stage, and declined longitudinally with distance downstream from the spillway outlet. The modelling approach used here provides an effective method for predicting likely outcomes of flexible hydropower operating conditions, taking into account fish species and life-stages present and watercourse characteristics.

Redefining the oceanic distribution of Atlantic salmon.

Determining the mechanisms driving range-wide reductions in Atlantic salmon marine survival is hindered by an insufficient understanding of their oceanic ecology and distribution. We attached 204 pop-up satellite archival tags to post-spawned salmon when they migrated to the ocean from seven European areas and maiden North American salmon captured at sea at West Greenland. Individuals migrated further north and east than previously reported and displayed increased diving activity near oceanographic fronts, emphasizing the importance of these regions as feeding areas. The oceanic distribution differed among individuals and populations, but overlapped more between geographically proximate than distant populations. Dissimilarities in distribution likely contribute to variation in growth and survival within and among populations due to spatio-temporal differences in environmental conditions. Climate-induced changes in oceanographic conditions will alter the location of frontal areas and may have stock-specific effects on Atlantic salmon population dynamics, likely having the largest impacts on southern populations.

The effects of hydrodynamics on the three-dimensional downstream migratory movement of Atlantic salmon.

Anthropogenic structures in rivers are major threats for fish migration and effective mitigation is imperative given the worldwide expansion of such structures. Fish behaviour is strongly influenced by hydrodynamics, but little is known on the relation between hydraulics and fish fine scale-movement. We combined 3D Computational fluid dynamics modelling (CFD) with 2D and 3D fish positioning to investigate the relation between hydrodynamics and the downstream movement of Atlantic salmon smolts (Salmo salar). We show that fish use fine-scale flow velocity and turbulence as navigation cues of fine-scale movement behaviour. Tri-dimensional swimming speed and swimming direction can be explained by adjustments of fish to flow motion, which are linked to fish swimming mode. Fish diverge from the flow by swimming at speeds within or higher than their prolonged speeds (0.38-0.73 m s-1). Flow direction plays a pivotal role on fish swimming performance, with high upstream and downwards velocities impacting swimming the most. Turbulence is also influential, by benefiting swimming performance at low TKE (< 0.03 m2 s-2) or constraining it at higher levels. We show that fish behaviour is affected by interactions of several hydraulic variables that should be considered jointly.

Modelling the effects of stranding on the Atlantic salmon population in the Dale River, Norway.

Rapid dewatering in rivers as a consequence of hydropower operations may cause stranding of juvenile fish and have a negative impact on fish populations. We implemented stranding into an Atlantic salmon population model in order to evaluate long-term effects on the population in the Dale River, Western Norway. Furthermore, we assessed the sensitivity of the stranding model to dewatered area in comparison to biological parameters, and compared different methods for calculating wetted area, the main abiotic input parameter to the population model. Five scenarios were simulated dependent on fish life-stage, season and light level. Our simulation results showed largest negative effect on the population abundance for hydropeaking during winter daylight. Salmon smolt production had highest sensitivity to the stranding mortality of older juvenile fish, suggesting that stranding of fish at these life-stages is likely to have greater population impacts than that of earlier life-stages. Downstream retention effects on the ramping velocity were found to be negligible in the stranding model, but are suggested to be important in the context of mitigation measure design.